It is well known that the operation of an integrated circuit generates heat. In many applications an appreciable amount of heat may be generated by the operation of an integrated circuit. In some instances the heat from the operation of an integrated circuit may be dissipated into the surrounding ambient atmosphere through convection. In other instances the amount of heat to be removed that is generated by an integrated circuit may require the use of an external cooling mechanism. An example of an external cooling mechanism is a fan that blows cooling air over an integrated circuit to carry away heat.
In some instances the amount of heat that is generated by an integrated circuit may require a relatively large amount of cooling in order to dissipate the generated heat. For example, a sustained operation of an integrated circuit in a high power application may require an increased level of cooling in order for the integrated circuit to function properly. There is therefore a need for an efficient heat removal process to cool an integrated circuit while the integrated circuit is operating.
Integrated circuit packages exist that remove heat from an integrated circuit die by conducting heat from the integrated circuit die into a thermally conductive slug. The heat absorbed in the thermally conductive slug is ultimately transferred to the ambient atmosphere or to some external heat sink. The thermally conductive slug may be formed from a thermally conductive metal such as copper.
For example, U.S. Pat. No. 5,642,261 to Bond et al. discloses an integrated circuit having a substrate in which an opening has been formed to receive a thermally conductive slug. The integrated circuit die is mounted on one side of the slug and the underside of the slug is exposed at the underside of the substrate. The integrated circuit die is wire bonded to the substrate and encapsulated in a conventional manner. Solder balls are attached to the underside of the substrate and of the slug in a ball grid array fashion for mounting on a circuit board. When the integrated circuit package is mounted on a circuit board, a path of high thermal conductivity is provided between the integrated circuit die and the circuit board through the slug and the solder balls.
In another example, U.S. Pat. No. 5,693,572 to Bond et al. discloses a method for mounting an integrated circuit die within an integrated circuit package in which the integrated circuit die is mounted on a thermally conductive slug. The slug is attached to an underside portion of a substrate through which an opening has been formed to receive the slug. The integrated circuit die is wire bonded to the substrate and encapsulated in a conventional manner. Solder balls are attached to the underside of the substrate in a ball grid array fashion for mounting on a circuit board. When the integrated circuit package is mounted on a circuit board, a path of high thermal conductivity is provided between the integrated circuit die and the circuit board through the slug. The circuit board serves as a heat sink.
In an alternate embodiment of the invention disclosed in U.S. Pat. No. 5,693,572 to Bond et al. the integrated circuit die is placed within the opening that is formed in the substrate and the slug is located below the plane of the underside of the substrate. The slug may be epoxied or otherwise securely fastened to lateral surfaces of the underside of the substrate. Such integrated circuit packages are also the subject of U.S. Pat. No. 5,991,156 to Bond et al. Experience has shown that a very efficient method for attaching a thermally conductive slug of the type described above is a solder reflow process.
Semiconductor devices that are packaged in a ball grid array (BGA) package are usually mounted onto a circuit board using a surface mount process. During a surface mount process the temperature may exceed two hundred twenty degrees Celsius (220° C.). At this temperature the strength of a solder connection is reduced to zero for a solder alloy having a melting temperature that is less than two hundred twenty degrees Celsius (220° C.). It is possible to use a solder alloy that has a higher melting temperature but the use of such a solder alloy may cause irreversible damage to the substrate material.
Assume that a solder reflow process has been used to attach a thermally conductive slug to an integrated circuit die in a ball grid array package in the manner previously described in the prior art. During the surface mount process for mounting the ball grid array package to a circuit board, the high temperature causes the solder seal of the thermally conductive slug to be weakened. The expansion of moisture diffused into a cavity of the integrated circuit package causes an increase in vapor pressure within the cavity. The increased vapor pressure then causes the thermally conductive slug to shift or move out of its original position. Alternatively, the increased vapor pressure causes melted solder to ooze out from around the thermally conductive slug. The soldered thermally conductive slug may generally be referred to as a soldered lid.
There is therefore a need in the art for an efficient system for venting pressure from an integrated circuit package that is sealed with a soldered lid. Specifically, there is a need in the art for a system and method that is capable of providing a passage for moisture or vapor inside a cavity within a ball grid array integrated circuit package to escape from the cavity and reduce internal vapor pressure within the cavity.
Conductive epoxy can be used instead of solder to attach a lid to an integrated circuit die and substrate. However, if a continuous patch of epoxy material is applied the epoxy will stick to the integrated circuit die, the substrate and the solder mask, and will seal the cavity within the integrated circuit package. When this happens, an increase in vapor pressure caused by heating can cause an epoxied lid to move out of its position in the same manner as that previously described for a soldered lid. The term “epoxied lid” refers to a lid that is glued to another material using epoxy.
Therefore, there is also a need in the art for an efficient system and method for venting pressure from an integrated circuit package that is sealed with an epoxied lid.